Method of manufacture for an improved magnetoresistive read transducer

ABSTRACT

A magnetoresistive (MR) read transducer in which a layered structure comprising an MR layer, an antiferromagnetic material in direct contact with the MR layer and a thin layer of interdiffusion material in contact with the layer of antiferromagnetic material is subjected to a heating process to a temperature within a chosen temperature for a chosen time to form a magnetic interface between the antiferromagnetic material the MR layer. The magnetic interface produces a high level of exchange bias with the MR layer.

This is a divisional of U.S. patent application Ser. No. 07/779,221filed Oct. 18, 1991, U.S. Pat. No. 5,262,914.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates in general to magnetic transducers for readinginformation signals from a magnetic medium and, in particular, to animproved magnetoresistive read transducer and a method for making theimproved transducer.

2. Description of the Prior Art

The prior art discloses a magnetic transducer referred to as amagnetoresistive (MR) sensor or head which has been shown to be capableof reading data from a magnetic surface at great linear densities. An MRsensor detects magnetic field signals through the resistance changes ofa read element made from a magnetoresistive material as a function ofthe amount and direction of magnetic flux being sensed by the element.

The prior art also teaches that in order for a MR sensor to operateoptimally, two bias fields should be provided. In order to bias thematerial so that its response to a flux field is linear, a transversebias field is generally provided. This bias field is normal to the planeof the magnetic media and parallel to the surface of the planar MRsensor.

The other bias field which is usually employed with MR sensors isreferred to in the art as the longitudinal bias field, which extendsparallel to the surface of the magnetic media and parallel to thelengthwise direction of the MR sensor. The function of the longitudinalbias field is to suppress Barkhausen noise, which originates frommulti-domain activities in the MR sensor.

A MR sensor for reading information signals from a magnetic recordingmedium is described in U.S. Pat. No. 4,103,315 to Hempstead, et al.,which is assigned to the same assignee as this application. The '315patent describes a MR read sensor which utilizesantiferromagnetic-ferromagnetic exchange coupling to produce a uniformlongitudinal bias in the MR layer of the sensor. The materials suggestedby the '315 patent are nickel-iron (NiFe) as the ferromagnetic MR layerand a manganese (Mn) alloy as the antiferromagnetic layer. Of thepossible Mn alloys, iron-manganese (FeMn) appears to exhibit thegreatest ability to exchange couple with the NiFe layer, and the FeMn isdeposited directly on the NiFe to obtain the exchange bias effect. Thestrength of the exchange bias field developed by the materials suggestedin the '315 patent was sufficient to meet prior art requirements.However, the drive to increased recording density has led to therequirement for greater levels of exchange bias field.

The use of a thermal treatment to produce a new ternaryantiferromagnetic material by diffusion between contacting layers ofNiFe and FeMn is described in U.S. Pat. No. 4,809,109. This methodproduces a higher level of exchange bias field and an increase in theordering temperature of the antiferromagnetic material. However, thismethod is not compatible with prior art manufacturing processes for thinfilm magnetic heads.

The prior art does not disclose an MR head which produces a high levelof exchange bias which can be produced by a process which is compatiblewith prior art manufacturing processes for thin film magnetic heads.

SUMMARY OF THE INVENTION

It is therefore the principal object of this invention to increase theexchange bias magnitude of a magnetoresistive (MR) read transducer.

In accordance with the invention, the objective is achieved by providinga thin MR layer of ferromagnetic material, a thin layer ofantiferromagnetic material in direct contact with the MR layer, and athin layer of an interdiffusion material in contact with the layer ofantiferromagnetic material. This layered structure is subjected toheating to a temperature within a predetermined range for apredetermined time to form a magnetic interface between the layer ofantiferromagnetic material and the MR layer. This magnetic interfacematerial produces a high exchange bias field with the MR layer.

In a specific embodiment the MR layer comprises NiFe, the layer ofantiferromagnetic material comprises MnFe, the interdiffusion materialis gold and the heating step comprises heating to a temperature of about240° C. for a time of about 7 hours. This method is fully compatiblewith prior art manufacturing processes for thin film magnetic heads.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention as illustrated inaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an end view of a specific embodiment of a magnetoresistiveread transducer according to the present invention.

FIG. 2 is an end view of an alternate embodiment of a magnetoresistiveread transducer according to the present invention.

FIG. 3 is a graph of sheet resistance as a function of process annealingtreatments for a series of layered structures in which the thickness ofa tantalum capping layer is varied from 0 to 15 nm.

FIG. 4 is a graph of exchange bias field as a function of processannealing treatments for a series of layered structures in which thethickness of a tantalum capping layer is varied from 0 to 15 nm.

FIG. 5 is a graph of exchange bias, normalized to 300 Å NiFe, as afunction of process annealing treatments for the series of layeredstructures shown in FIG. 4.

FIG. 6 is a graph of exchange bias field H_(ua) as a function oftemperature for layered structures with and without a tantaluminterlayer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A specific embodiment of a magnetoresistive (MR) sensor will bedescribed briefly in conjunction with FIG. 1. The MR sensor 10 comprisesa thin MR layer of ferromagnetic material 12 which extends only over thecentral active region 11. A thin layer of a suitable antiferromagneticmaterial 14 is deposited in good interfacial contact with the MR layer12. A resultant unidirectional anisotropy develops by means of exchangecoupling across the interface between MR layer 12 and antiferromagneticlayer 14 and produces a shift of the MH loop of the MR layer which isusually called the longitudinal exchange bias field H_(ua). Thetransverse bias can be produced by a soft magnetic film layer (notshown) which is separated from the MR layer 12 by a thin nonmagneticspacer layer (not shown) whose purpose is to prevent magnetic exchangebetween the MR layer 12 and the soft magnetic bias layer as is known inthe art. Conductor leads 16 are included for connection to a sensingmeans (not shown) for sensing an output signal as is know in the art. Inthe embodiment shown, conductor leads 16 comprise a layered structurecomprising a thin layer of tantalum (Ta) 20, a layer of a good conductor22, such as gold (Au) and a thin capping layer of Ta 24.

In the embodiment shown in FIG. 2, a different metallurgy is used forconductor leads 16' which comprises a layer of a good conductor 26.Depending on the properties of the chosen material for conductor 26, alayer of either adhesion and/or diffusion barrier 28 may be required.

According to the present invention, the exchange bias for theMR/antiferromagnetic couple films is substantially increased by theaddition of a layer of interdiffusion material 18 which is deposited incontact with the layer of antiferromagnetic material 14.

In a specific embodiment the preferred material for the MR layer 12 isNiFe and the preferred material for the antiferromagnetic layer 14 isMnFe. The layer of interdiffusion material is preferably a noble metalsuch as gold, rhodium, platinum, palladium, or ruthenium. However, otherelements, such as copper and silicon, which are readily interdiffusablewith MnFe, are possible candidates for this application.

To demonstrate the principle of the present invention, a layeredstructure was built to vary the amount of interdiffusion between thelayer of interdiffusion material 18 and the antiferromagnetic layer 14.The films comprising the layered structure were sputter deposited ontoglass in the following configuration:

    Ta (200 Å)/Au (600 Å)/Ta (thickness varied)/MnFe (240 Å)/NiFe/

glass, and the layered structures were subjected to a plurality of thestandard vacuum annealing cycles (240° C., 7 hours). The results areshown in FIG. 3 where it can be seen that the structure without the Taspacer layer shows a substantial increase in sheet resistance upon onlyone annealing cycle and reached a steady state with more annealingcycles. Auger depth profiling confirms that interdiffusion occurred at240° C. rapidly between Au and MnFe. FIG. 3 also illustrates that a verythin Ta film of 40 Å or thicker is effective to suppress theinterdiffusion.

The exchange bias of the same structures as a function of the annealingcycles is shown in FIGS. 4 and 5. FIG. 4 shows the exchange bias vs.standard vacuum annealing cycle for the structures with various Tainterlayer thickness. It can be seen that for the structure with theMnFe in direct contact with the Au, the exchange bias increasesdramatically upon annealing. The exchange bias value increased from 28.8Oe for the as-deposited film to a value of 49.1 Oe after nine annealingcycles. The other structures with a Ta interlayer thickness of more than40 Å showed only modest increases in exchange bias. This clearly showsthat interdiffusion between Au and MnFe plays an important role ofenhancing the exchange bias value.

This is even more clearly shown in FIG. 5 which shows the normalizedexchange bias for the same structures shown in FIG. 4. Note in thelegend at the upper left of FIG. 4 that the thickness of the NiFe layersvaries. For comparison, the exchange bias normalized to 300 Å NiFethickness vs. annealing cycle is shown in FIG. 5. The normalizedexchange bias increased from 34 Oe (as-deposited film) to 61.7 Oe afterfive vacuum annealing cycles for the structure without a Ta interlayer,while the rest of the structures with various Ta interlayer thicknessesshow only very slight increase.

The microstructure origins to these exchange bias enhancements are notclearly understood. It is clear that a thermally formed magneticinterface is formed between the antiferromagnetic material 14 and the MRlayer 12. The magnetic interface is that interface at which thematerials system changes from a ferromagnetic character on one side toan antiferromagnetic character on the other side. The alteredcomposition at the magnetic interface may comprise a ternary or otherantiferromagnetic composition.

The enhancement in exchange bias was observed for other Au filmthickness as well. Table I shows the results of three different Authicknesses ranging from 200 Å to 1200Å.

                  TABLE I                                                         ______________________________________                                        Normalized (NiFe = 300Å) Exchange Bias                                              Au(200Å)                                                                            Au(600Å)                                                                            Au(1200Å)                                   ______________________________________                                        MnFe/NiFe   28.3 Oe     29.9 Oe   28.4 Oe                                     As - Dep.                                                                     Au/Ta       33.3 Oe     34.8 Oe                                               As - Dep.                                                                     1           44.4 Oe       50 Oe   45.9 Oe                                     Anneal Cycle                                                                  2           48.6 Oe     54.1 Oe   49.7 Oe                                     Anneal Cycles                                                                 3           50.7 Oe     59.1 Oe   50.7 Oe                                     Anneal Cycles                                                                 4           56.7 Oe     57.1 Oe   52.4 Oe                                     Anneal Cycles                                                                 5           59.3 Oe     59.6 Oe   54.2 Oe                                     Anneal Cycles                                                                 6                       62.9 Oe                                               Anneal Cycles                                                                 ______________________________________                                         All films capped with 200Å Ta.                                       

The results shown in Table I demonstrate that the observed enhancementin exchange bias is operational over a wide range of gold thickness.

The temperature dependences of the exchange bias normalized to 300 ÅNiFe for the layered structures with and without the Ta interlayer areshown in FIG. 6 is in terms of cycles of heating at 240° C. for about 7hours. However, a temperature within the range of 220° to 320° C. and atime within the range of 2 to 12 hours is also suitable. Thesestructures, for which the data of FIG. 6 are shown, are the same as thefirst and fourth structures shown in FIG. 4. Both structures had beenthrough five vacuum anneal cycles before measurement. Five cycles waschosen to simulate the treatment in a particular head fabricationprocess. The blocking temperature, inferred from the data as theintercept of the linear fit with 0 Oe, was 157° C. for the structurewith 150 Å Ta interlayer, while the structure with no Ta interlayer hada blocking temperature of 153° C.

At 80° C., the normal operating temperature of the magnetic head, thenormalized exchange bias is 35 Oe in the case where the Au is in directcontact with the MnFe and 21.1 Oe for the structure with the Tainterlayer which is a difference of 14 Oe.

In view of the above reported data, the preferred interdiffusionmaterial is Au, and the preferred thickness for this layer is 200 Å. Thepreferred MR layer is formed of NiFe, and the preferredantiferromagnetic material in MnFe. With the use of these materials, thebenefits of the thermally induced interdiffusion techniques can beobtained without adding any additional complexity to an already existingmanufacturing process. The Au layer is deposited directly onto the MnFelayer and the lead metallurgy is deposited directly in the same vacuumpump-down.

A process has been disclosed by which a much higher exchange bias can beachieved with a blocking temperature that is substantially the same asprior art structures. Improved corrosion resistance also results fromthis process. This process can be applied to make greatly improved MRread transducers without adding more steps or complexity to themanufacturing process.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various other changes in the form anddetails may be made therein without departing from the spirit and scopeof the invention.

We claim:
 1. A method for making a magnetoresistive read transducerassembly comprising the steps of:providing a substrate; depositing athin magnetoresistive layer of ferromagnetic material on said substrate;depositing a thin layer of antiferromagnetic material in direct contactwith the magnetoresistive layer; depositing a thin layer of aninterdiffusion material in contact with the layer of antiferromagneticmaterial; and heating said layered structure to form a magneticinterface between said antiferromagnetic material and saidmagnetoresistive layer whereby said magnetic interface produces anexchange bias field in said magnetoresistive layer.
 2. The method ofclaim 1 wherein said interdiffusion material comprises a noble metal. 3.The method of claim 2 wherein said noble metal is taken from the groupconsisting of gold, rhodium, platinum, palladium, and ruthenium.
 4. Themethod of claim 1 wherein said magnetoresistive layer comprises an alloyof nickel and iron and said antiferromagnetic layer comprises an alloyof iron and manganese.
 5. The method of claim 4 wherein saidinterdiffusion material comprises a noble metal.
 6. The method of claim5 wherein said noble metal is taken from the group consisting of gold,rhodium, platinum, and palladium, and ruthenium.
 7. The method of claim6 wherein said noble metal is gold.
 8. The method of claim 7 whereinsaid heating step is conducted at a temperature within the range ofabout 220° to 320° C. for a time within the range of about 2 to 12hours.
 9. The method of claim 8 wherein said heating step is conductedat a temperature of about 240° C. for a time of about 7 hours.